46 THE QUANTITATIVE METHOD IN BIOLOGY 



Innumerable similar examples are found in plants and also in 

 animals. 



Segregation in the Course of Individual Development. — Accord- 

 ing to one of the laws of heredity laid down by DARWIN, 

 variations are inherited at a corresponding age.^ 



Several important consequences follow from this law. A 

 direct consequence is that all the primordia of a species are not 

 observed simultaneously, but successively. This is a cause of 

 segregation. The study of individual evolution is therefore a 

 method of discovering the primordia and of decomposing the 

 compound properties into their components, just as the in- 

 vestigation of the Mendelian segregation. 



The last period of the individual development (which has been 

 hitherto rather neglected by embryologists) is especially inter- 

 esting for the analysis of those properties by which subspecies, 

 species and even genera are characterised. In this paragraph 

 I limit myself to one example. (See Myosotis, § 46, and Cen- 

 taur ea, § 47.) 



A subspecies of Viola tricolor which resembles V. lutea is very 

 common on the dunes along the Flemish coast. When the 

 floral bud expands the five petals are on the whole pale yellow. 

 After a certain time a blue pigment appears in the two upper 

 petals, in which the yellow colour is no longer visible. The 

 difference between young and older flowers is very striking. 

 Suppose now that a Viola with blue upper petals has been 

 crossed with a Viola with yellow upper petals, and that the 

 properties are transmitted in the typical Mendelian way. In 

 the second filial generation Fg one would observe two sorts of 

 specimens : blue and yeUow. The difference would be exactly 

 the same as between a young and an older flower in the 

 above example. (On the relation between development and 

 gradation, see § 133.) 



Conclusion. — From the examples mentioned in the present 

 paragraph it is seen that segregation (with regard to the observ- 

 able properties) may be produced by four different causes : 

 hybridization, plasticity, gradation and succession in the course 

 of development. From this it may be concluded that segrega- 

 tion of the observable properties is in general independent of 

 hybridization. In other words, segregation of the properties 

 ought to be looked upon as being a something which has its 

 proper existence, dependent in each peculiar case on a specific 

 energy of the primordia under consideration, and which is 

 not necessarily a result of the presence or absence of certain 

 hereditary determiners (factors). 



The analysis of the properties by the study of segregation is 



^ DARWIN, The Variation of Animals and Plants under Domestication 

 (second edition, 1875), vol. ii., pp. 51-61. 



